An image film has a sound signal recording portion, along with contiguous image frames and a large number of perforations formed at a fixed interval for extending along both lateral sides along the film width for positively feeding the image frames. On this sound signal recording portion are recorded sound signals in association with the video frames.
In a recent imaging system, a so-called digital sound projection system is adopted, in which not only the analog sound but also the digital sound is used for achieving higher sound quality and improved theater-presence feeling by the stereo sound. The image film for the digital sound projection system is provided with a digital sound recording portion, in addition to an analog sound recording portion, in order to make up the sound signal recording portion by the analog and digital sound recording portions.
That is, the image film for the digital sound system is provided with the digital sound recording portions having the digital sound signals recorded thereon. Each of these digital sound recording portions is formed in association with contiguous image frames between the left-side row of perforations and the film edge and between right-side row of perforations provided at a spacing from each other along the film feed direction. These digital sound signals are read out and reproduced by a digital sound reproducing device 100 which will be explained subsequently.
The digital sound signal reproducing device 100 is constituted by a first guide roll 101, a guide sprocket 102, a sound drum 103 and a second guide roll 104. These components are combined with the digital projection device and constitute a film running path for an image film supplied from an automatic image film supply takeup device, not shown. The digital sound signal reproducing device 100 also includes first to fourth tension rolls 105 to 108 in order to permit the image film 1 to run in stability along the running path components. An optical readout device 120 for the digital sound signals as later explained is annexed to the sound drum 103.
A tension spring 109 for biasing the image film 1 against the guide sprocket 102 and the sound drum 103 by its resilient force is installed under tension across the second tension roll 106 and the third tension roll 107.
The image film 1 is reeled out from the automatic image film supply takeup device and thence supplied to an image film supply port of the digital sound signal reproducing device 100 so as to be placed around the first guide roll 101 and so as to be turned back in the horizontal direction. The image film 1 is led to the sound drum 103 via second tension roll 106 as the film is thrust by the first tension roll 105 against the guide sprocket 102. The image film 1 is turned back by this sound drum 103 so as to be placed around the second guide roll 104 as the film is thrust by the second tension roll 107 and the fourth tension roll 108 against the guide sprocket 102. The image film 1 is fed out at the image film outlet port of the digital sound signal reproducing device 100 via second guide roll 104 and thence supplied to a driving sprocket 111 operating as a driving source for entraining the film to the digital sound projection device.
The guide sprocket 102 is provided with a flywheel 110 coaxially with the axis of the guide sprocket 102, as shown by a dotted line in FIG. 1.
In the vicinity of the sound drum 103 is mounted an optical readout device 120 for digital sound signals. The optical readout device is made up of a readout light source, not shown, an optical fiber 121, an objective lens device 122 and a CCD line sensor 123.
In the optical readout device 120 for digital sound signals, a light beam radiated from a light source is led into the optical fiber 121 so as to be radiated to a digital sound recording portion of the image film 1 to form an image on a CCD line sensor 123 via an objective lens 122 in the form of optically bright and dark bar-code signals converted from the light beam.
Meanwhile, in the above-described digital sound projection system, a digital projector and the automatic film supply takeup device are usually mounted at a spacing of several meters from each other. In addition, the digital sound signal reproducing device 100 has no driving source and supports the image film 1 with an extremely weak force in the inner image film running path. Thus, the image film 1 reeled from the automatic film supply device is suspended by its own gravity or wobbled under the effect of an extremely small wind force in a projection room. Thus the film travels in a pulsating unstable state on the image film path as indicated by a film portion M in FIG. 1.
This wobbling phenomenon of the image film 1 operates as fluctuations in the load applied on the image film 1 by the driving sprocket 111 via the digital sound signal reproducing device 100 thus producing rotational fluctuations or wow on the driving sprocket 111. Thus, with the conventional digital sound signal reproducing device 100, the image film 1 cannot be run in stability.
Meanwhile, in the digital sound signal reproducing device 100, the image film 1 is run in a stable state by running the guide sprocket 102 in a stable state by employing a flywheel 110 used for increasing the inertia of the guide sprocket 102, as shown by a broken line in FIG. 1. However, this flywheel 110, operating effectively when the image film 1 runs on the guide sprocket 102 in a stable state, cannot play a sufficient role in stable running of the image film if the driving sprocket 111 undergoes rotational fluctuations due to the wobbling phenomenon of the imaging film 1.
Thus, in the digital sound reproducing device 100, a problem is raised that digital sound signal clocks are produced such that readout of the digital sound signals recorded in the digital sound signal recording portion of the image film 1 fails to occur reliably at a constant timing in the optical readout device 120, thus causing the interrupted playback sound.
For overcoming this drawback, it may be contemplated to provide in the above-described digital sound reproducing device 100 driving units at the image film supply port and at the image film outlet port synchronously rotated for driving the imaging film 1 for preventing the running in a pulsating unstable state of the image film 1.
However, with this digital sound signal reproducing device, there arises a necessity of driving the driving unit in reliably synchronism with the automatic film supply takeup device or the driving sprocket o the digital projection device, thus complicating and increasing the size of the main body portion of the device. Moreover, since the driving unit is provided in the digital sound signal reproducing device, there arises the necessity of modifying and newly constructing the basic specifications of component devices of the digital sound projection system.
For overcoming the above drawbacks, it may be contemplated to increase the tension applied to the tension rolls 105 to 108 provided in the digital sound signal reproducing device 100 in order to permit the image film 1 to run in a perpetually taut state. With this digital sound signal reproducing device, since the image film 1 is perpetually run under a large tension, a driving force more powerful than the driving sprocket is required, thus giving rise to the necessity of modifying and newly constructing the basic specifications of the component parts of the digital sound projection system.
In addition, if the imaging film 1 is run under perpetually large tension on the film running path, film perforations tend to be deformed and destroyed. On the other hand, the film surface tends to be damaged to reduce the service life of the film.
It is therefore an object of the present invention to provide a digital sound signal reproducing device whereby the image film can be run in stability to improve reliability in reproduction and to simplify the structure of the main body portion of the device.